As we age, the chemical signature of our DNA changes subtly, with genes becoming more or less methylated (that is, they pick up or lose certain methyl chemical groups) in a process known as epigentics. Now, Steve Horvath at the University of California, Los Angeles has used that knowledge to estimate age, reports New Scientist.

They took 7,844 healthy tissue samples from 51 different types of tissue, with participant age ranging from fetus to 101. Then, they analyzed how methylation varied between site, allowing them to identify a subset of 353 regions of the genome that became either more or less methylated with age in almost all types of tissue. In turn, that can be flipped on its head to indicate the apparent age of tissue—a technique they validated against thousands of other samples.

The interesting upshot is that some parts of the body seem to age quicker than others. Healthy breast tissue, for example, was on average two to three years older than the woman's actual age. Elsewhere, heart tissue in both sexes appears nine years younger than true age. The research is published in Genome Biology.

It's not just a strange scientific quirk, though. Because the technique can be used on blood, the researchers claim it could be used forensically, to reveal the age of a murder suspect, say. [Genome Biology via New Scientist]